Method of preparing metal oxide-silica composite aerogel and metal oxide-silica composite aerogel prepared by the same

ABSTRACT

Provided are a method of preparing a metal oxide-silica composite aerogel, and a metal oxide-silica composite aerogel having an excellent weight reduction property prepared by the method. The method includes a step of adding an acid catalyst to a first water glass solution to prepare an acidic water glass solution (step 1); a step of adding a metal ion solution to the acidic water glass solution to prepare a precursor solution (step 2); and a step of adding a second water glass solution to the precursor solution and performing a gelation reaction (step 3) to yield a metal oxide-silica composite wet gel, wherein, in steps 2 and 3, bubbling of an inert gas is performed during the adding of the metal ion solution or the second water glass solution, respectively.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a National Stage Application of International Application No. PCT/KR2018/010520 filed on Sep. 7, 2018, which claims the priority benefit of Korean Patent Application No. 10-2017-0115276, filed on Sep. 8, 2017 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference. The present invention relates to a method of preparing a lightweight metal oxide-silica composite aerogel and a metal oxide-silica composite aerogel prepared by the same.

BACKGROUND

A silica aerogel, which is a super-porous and high-specific-surface-area material having a porosity of about 90 to 99.9% and a pore size of 1 to 100 nm, has excellent characteristics such as ultra-lightweight/super insulation/super low dielectricity. Accordingly, research into the development of an aerogel material and application thereof to transparent insulating materials, eco-friendly high-temperature-resistance insulating materials, extremely low dielectric thin films for highly integrated devices, catalysts, catalyst carriers, electrodes for super-capacitors, electrode materials for seawater desalination, and the like is actively underway.

The greatest advantage of a silica aerogel is super-insulation characterized by a thermal conductivity of 0.300 W/mK or less, which is lower than thermal conductivities of conventional organic insulating materials such as Styrofoam. In addition, problems, such as fire vulnerability and generation of harmful gases in the case of fire, which are fatal disadvantages of organic insulating materials, can be addressed.

Meanwhile, to further increase a utilization range of a silica aerogel, a method of adding mechanical properties to original properties of the silica aerogel has been studied. For example, a metal oxide-silica composite aerogel into which a metal oxide is introduced has been developed.

Conventional general processes of preparing a metal oxide-silica composite aerogel are disadvantageous in that they are complicated, a large amount of wastewater is generated due to the use of an organic solvent when the solvent is replaced, and an excessive amount of surface modifier is consumed in a surface modification step.

To address the problems, research into a low-cost production process in which a high-cost surface substitution process and pyrolysis process requiring an organic solvent and a surface modifier are omitted has recently been conducted.

In particular, research into a method of preparing a metal oxide-silica composite aerogel, the method including a step of adding a metal ion solution and an acid catalyst to a water glass solution and reacting the same (step 1); and a step of washing and drying a wet gel (step 2), has been conducted.

However, a metal oxide-silica composite aerogel prepared by the method has a broad pore distribution due to a non-uniform reaction, caused by the formation of a composite structure of a metal oxide and silica, and a low effective pore volume due to severe contraction during drying caused by omission of a surface modification reaction of stabilizing particle surfaces.

A metal oxide-silica composite aerogel has an open-pore structure due to the formation of a network structure among primary particles thereof and is lightweight due to the limitation of penetration of a polymer resin into the pore. The volume of the effective pores, into which penetration of the polymer resin is limited, is an important factor in using the metal oxide-silica composite aerogel as an additive for weight reduction. However, since such a metal oxide-silica composite aerogel has a low effective pore volume as described above, the properties thereof as an additive for weight reduction are poor.

In addition, the metal oxide-silica composite aerogel suffers a great particle size change when shear pressure from the outside is applied thereto, due to a poor network structure thereof. In particular, since a particle size of the metal oxide-silica composite aerogel is greatly reduced, a particle structure thereof collapses due to shear pressure, during extrusion or injection-molding of a polymer resin, when applied as an additive to the polymer resin. Accordingly, the applicability thereof as an additive is significantly decreased.

Therefore, there is an urgent need for a process of preparing a metal oxide-silica composite aerogel capable of improving the uniformity of a reaction between a metal oxide and silica to have a narrow pore distribution, strengthening an internal network structure to reduce contraction during drying and, accordingly, improving pore volume and effective density, and reducing the rate of particle size change due to shear pressure to allow application as an additive for weight reduction.

DISCLOSURE Technical Problem

Therefore, the present invention has been made in view of the above problems, and it is one object of the present invention to provide a method of preparing a metal oxide-silica composite aerogel having a more uniform pore structure, a strengthened network structure, and an excellent weight-lightening property due to an increased pore volume.

It is another object of the present invention to provide a metal oxide-silica composite aerogel prepared by the method.

Technical Solution

In accordance with an aspect of the present invention, the above and other objects can be accomplished by the provision of a method of preparing a metal oxide-silica composite aerogel, the method including a step of adding an acid catalyst to a first water glass solution to prepare an acidic water glass solution (step 1); a step of adding a metal ion solution to the acidic water glass solution to prepare a precursor solution (step 2); and a step of adding a second water glass solution to the precursor solution and performing a gelation reaction (step 3), wherein, in steps 2 and 3, bubbling is performed with an inert gas.

In accordance with another aspect of the present invention, there is provided a metal oxide-silica composite aerogel prepared by the method.

Advantageous Effects

A method of preparing a metal oxide-silica composite aerogel according to the present invention can reduce production costs due to omission of a surface substitution process and a pyrolysis process, thus providing high economic efficiency. In addition, the method can improve the uniformity of a reaction between a metal oxide and silica. Further, the method can strengthen an internal network structure, thereby suppressing contraction during drying and thus effectively preventing the collapse of pore structures.

In addition, since a metal oxide-silica composite aerogel prepared by the method according to the present invention has a uniform pore structure and a further strengthened network structure, pore characteristics, such as a specific surface area, a pore volume, a pore uniformity, an effective pore volume, and the effective density of single particles, of the metal oxide-silica composite aerogel can be remarkably improved. Accordingly, the metal oxide-silica composite aerogel can be ultra-lightweight.

Further, since a metal oxide-silica composite aerogel prepared by the method according to the present invention is subjected to primary particle growth inhibition and network structure strengthening by performing bubbling with an inert gas until a second water glass solution is added and a gelation reaction is performed and completed from when a metal ion solution is added, the rate of particle size change thereof due to shear pressure is very small and it has a very high particle strength. Accordingly, the metal oxide-silica composite aerogel can be usefully used as an additive for an extruded or injection-molded polymer resin.

Therefore, the method according to an embodiment of the present invention and a metal oxide-silica composite aerogel prepared by the method can be usefully used in related industries, e.g., silica aerogel-related industries. In particular, a metal oxide-silica composite aerogel prepared by the method can be used as an additive for weight reduction.

DESCRIPTION OF DRAWINGS

The attached drawings are provided to exemplify particular embodiments of the present invention and assist understanding of the technical idea of the present invention along with the aforementioned description, and the present invention should not be limited to particulars described in the drawings.

FIG. 1 is a flowchart schematically illustrating a conventional general method of preparing a metal oxide-silica composite aerogel.

FIG. 2 is a flowchart schematically illustrating a method of preparing a metal oxide-silica composite aerogel according to an embodiment of the present invention.

FIG. 3 is a graph illustrating a pore diameter-dependent distribution of a metal oxide-silica composite aerogel prepared according to examples of the present invention.

FIG. 4 is a graph illustrating a pore diameter-dependent distribution of metal oxide-silica composite aerogels prepared according to an example of the present invention and comparative examples.

FIG. 5 is a graph illustrating a pore diameter-dependent distribution enlarged based on the comparative examples of FIG. 4.

MODES OF THE INVENTION

Now, the present invention will be described in more detail to facilitate understanding of the present invention.

Terms or words used in the specification and the appended claims shall not be limited to common or dictionary meanings, and have meanings corresponding to technical aspects of the embodiments of the present invention so as to most suitably express the embodiments of the present invention.

A conventional general process of preparing a metal oxide-silica composite aerogel is complicated because it includes an aging step, a solvent replacement step, and a surface modification step and has problems such as a large amount of wastewater generated due to the use of an organic solvent when the solvent is replaced, and use of a large amount of surface modifier in the surface modification step.

To address the problems, a low-cost production process, in which a high-cost surface substitution process and pyrolysis process requiring an organic solvent and a surface modifier are omitted, as shown in FIG. 1 has been proposed. However, a metal oxide-silica composite aerogel prepared by the method has a broad pore distribution due to a non-uniform reaction, caused by the formation of a composite structure of a metal oxide and silica, and a low effective pore volume due to severe contraction during drying caused by omission of a surface modification reaction of stabilizing particle surfaces. Accordingly, the performance of the metal oxide-silica composite aerogel is too poor for use as an additive for weight reduction.

Therefore, the present invention provides a method of preparing a metal oxide-silica composite aerogel capable of addressing conventional problems, having a uniform pore structure and a strengthened network structure, and an improved specific surface area, pore volume, effective density, and the like.

Hereinafter, a method of preparing a metal oxide-silica composite aerogel according to an embodiment of the present invention is described in detail with reference to FIG. 2.

First, FIG. 2 is a flowchart illustrating a method of preparing a metal oxide-silica composite aerogel according to an embodiment of the present invention. The method includes a step of adding an acid catalyst to a first water glass solution to prepare an acidic water glass solution having a low pH (step 1); a step of adding a metal ion solution to the acidic water glass solution to prepare a precursor solution (step 2); and a step of adding a second water glass solution to the precursor solution and performing a gelation reaction (step 3). Here, steps 2 and 3 include a step of bubbling with an inert gas. A high concentration second water glass solution is slowly added to the precursor solution to induce a slow gelation reaction, thereby preparing a gelated material having a strengthened network structure. As such, since the method of preparing a metal oxide-silica composite aerogel according to the present invention does not include a solvent replacement process or a surface modification and a pyrolysis process, a process of the method is relatively simple, thereby increasing production efficiency. In addition, the method can reduce production costs, thereby increasing economic efficiency. In addition, a composite aerogel prepared according to the method has improved mechanical properties due to excellent uniformity of a reaction between a metal oxide and silica. In particular, due to a further strengthened network structure of the composite aerogel, the volume of effective pores, which inhibit penetration of a polymer resin, and an effective density are improved, and pore characteristics, a specific surface area, and the like are significantly improved. Accordingly, the composite aerogel can be used usefully as an additive for weight reduction.

Further, by performing bubbling with an inert gas, a metal ion solution and a precursor solution can be uniformly mixed, a dissolved oxygen amount in a solution can be lowered, and a further strengthened network structure can be formed by inhibiting the growth of primary particles during a gelation reaction. Accordingly, the mechanical properties of a resultant metal oxide-silica composite aerogel and properties thereof improved by the formation of a metal oxide can be significantly improved.

Here, a metal oxide-silica composite wet gel can be obtained through steps 1 to 3.

“Water glass solution” in the terms, “first water glass solution” and “second water glass solution,” used in the present invention can be a dilute solution prepared by adding distilled water to water glass and mixing the same. The water glass may be sodium silicate (Na₂SiO₃) as an alkali silicate obtained by melting silicon dioxide (SiO₂) and an alkali. The “first” and “second” can be provided to distinguish the order of addition. That is, the “first water glass solution” and “second water glass solution” can indicate water glass solutions sequentially added. In addition, as needed, the “first water glass solution” and “second water glass solution” can be used to indicate different water glass concentrations in the water glass solutions.

In the present invention, “gelation reaction” indicates a sol-gel reaction. The “sol-gel reaction” indicates a reaction of forming a network structure from silicon or a metal alkoxide unit precursor material, e.g., a reaction wherein reactants are connected to each other and grow through a condensation reaction wherein one water molecule of two functional hydroxy groups simultaneously forms a metal-oxygen bond, after a hydrolysis reaction of forming a functional hydroxy (—OH) group on a metal by reacting silicon with water.

Here, the network structure may be a planar net structure in which a specific polygon having one or more kinds of atomic arrays is connected, or a structure of sharing vertices, edges, and faces of a specific polyhedron to form a three-dimensional skeletal structure.

In the present invention, the term “precursor solution” refers to an acidic reactant solution that is prepared by mixing a metal ion solution with an acidic water glass solution prepared by adding an acid catalyst to a first water glass solution and thus includes metal ions. The precursor solution can be a sol-formulated precursor material as a reactant of a sol-gel reaction.

In the present invention, the term “average particle diameter (D₅₀)” is defined as a particle diameter corresponding to 50% of a cumulative number of particles in a particle size distribution curve. The average particle diameter (D₅₀) can be measured, for example, by a laser diffraction method. The laser diffraction method can generally enable measurement of a submicron to several-millimeter particle diameter and can provide a highly reproducible and resolvable result.

In the method according to an embodiment of the present invention, step 1 is a step of preparing an acidic water glass solution. In step 1, an acid catalyst is added to a first water glass solution, thereby preparing an acidic water glass solution.

The first water glass solution can have a concentration of 0.01 M to 2.0 M, preferably 0.05 M to 1.2 M, more preferably 0.1 M to 0.5 M. Here, the concentration of the first water glass solution means the concentration of water glass in the first water glass solution. That is, the first water glass solution can contain water glass within the concentration range.

When the concentration of the first water glass solution is less than 0.01 M, mixing is not uniformly performed when mixed with a metal ion in step 2 due to an excessively low concentration, whereby the uniformity of a reaction between a metal oxide and silica may be decreased. When the concentration of the water glass is greater than 2.0 M, a large amount of acid catalyst is used to maintain an acidic water glass solution due to high basicity, whereby economic efficiency is low in terms of production efficiency and production costs. In addition, since the second water glass solution should be added in a large amount for a pH suitable for reaction when the second water glass solution is added, the properties of a prepared metal oxide-silica composite aerogel may be deteriorated. When the second water glass solution is added in a suitable amount, not an excess amount, addition of a basic catalyst may be required, which may complicate the process and cause an economic efficiency decrease in terms of production efficiency and production costs.

In step 1, the pH of the acidic water glass solution can be 0.1 to 2.9, preferably 0.3 to 2, more preferably 0.5 to 1.5. The pH can be adjusted using an acid catalyst.

pH, which indicates the concentration of hydrogen ions or acidity, can be used as a numerical value indicating the degree of acidity or basicity of a material and can be an inverse value of a logarithm of the dissociation concentration of hydrogen ions.

When the pH of the acidic water glass solution is higher than 2.9, a gelation reaction of the first water glass solution is performed, whereby an aggregate can be formed. In addition, a precipitation reaction can occur when the metal ion solution is added in step 2, whereby an aggregate mixed with metal ions can be generated. The aggregate can be a gelated material or a precipitate.

When a gelation reaction of the first water glass solution is performed and thus an aggregate is formed, a prepared aerogel may have a weak network structure and large pores, whereby contraction may increase during drying and thus pore characteristics may be deteriorated. In addition, the uniformity of a reaction with a metal oxide may be significantly decreased, whereby the mechanical properties and the like of a prepared aerogel may be deteriorated. In addition, also when an aggregate mixed with metal ions is formed, the metal oxide and silica in the water glass solution are not uniformly mixed, whereby pore characteristics, mechanical properties, and the like of a prepared aerogel may be deteriorated.

Here, an added acid catalyst type is not specifically limited and can be, for example, one or more selected from the group consisting of hydrochloric acid, nitric acid, acetic acid, sulfuric acid, and hydrofluoric acid, preferably hydrochloric acid. An acid catalyst content in the acidic water glass solution is not specifically limited and can be included in an amount at which the pH of the acidic water glass solution is within the above range.

In the method according to an embodiment of the present invention, step 2 is a step of preparing a precursor solution. The precursor solution can be prepared by adding a metal ion solution and mixing the same while bubbling an acidic water glass solution with an inert gas.

Here, the bubbling can be performed using an inert gas, particularly any one of helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), radon (Rn), nitrogen (N₂), and carbon dioxide (CO₂) gases, more particularly a nitrogen gas.

As such, by performing bubbling with an inert gas when metal ions are added, a dissolved oxygen amount of a precursor solution can be lowered. When the dissolved oxygen amount of the precursor solution is low, a network structure of a metal oxide-silica composite aerogel prepared after the gelation reaction in the subsequent step 3) can be further strengthened. Particles, the network structure of which is strengthened, show a very uniform pore distribution and high particle strength, which can improve the rate of particle size reduction caused by breakage and distortion of particles due to shear pressure. That is, when the metal oxide-silica composite aerogel of the present invention is applied as an additive for a polymer resin, collapse of the structure thereof due to shear pressure can be suppressed although the metal oxide-silica composite aerogel is subjected to a molding process such as extrusion or injection-molding along with the polymer resin. Accordingly, the metal oxide-silica composite aerogel can be usefully used as an additive.

The metal ion solution can be prepared by dissolving a metal salt, as a raw material, in a solvent such that a metal oxide is formed in a finally produced metal oxide-silica composite aerogel. The concentration of the metal ion solution can be 0.01 M to 2.0 M, preferably 0.01 M to 1.2 M, more preferably 0.01 M to 0.2 M. Here, the concentration of the metal ion solution means the concentration of metal ions in the metal ion solution.

When the concentration of the metal ion solution is less than 0.01 M, silica may not be uniformly mixed in the water glass solution and pore characteristics of a prepared aerogel may be deteriorated, due to an excessively low concentration. In addition, due to an excessively low content of the metal oxide in the aerogel, mechanical properties and the like may not be improved. When the concentration of the metal is greater than 2.0 M, the porous structure of the prepared aerogel may be impaired, whereby properties in terms of a pore volume may be deteriorated.

A solvent for the metal ion solution is not specifically limited so long as a metal salt can be sufficiently dissolved, and can be, for example, distilled water.

The metal ion solution can include particularly ions of one or more selected from the group consisting of alkali metals, alkaline earth metals, lanthanides, actinides, transition metals, and metals in Group 13 (IIIA), more particularly ions of one or more selected from the group consisting of calcium (Ca), magnesium (Mg), copper (Cu), zinc (Zn), manganese (Mn), cadmium (Cd), lead (Pb), nickel (Ni), chromium (Cr), silver (Ag), titanium (Ti), vanadium (V), cobalt (Co), molybdenum (Mo), tin (Sn), antimony (Sb), strontium (Sr), barium (Ba), and tungsten (W). The metal salts forming metal ions can be appropriately selected and used according to the use of the metal oxide-silica composite aerogel. In the present invention, the metal salt can preferably include calcium and magnesium as metals.

In addition, the metal salt can be one or more metal salts selected from the group consisting of a chloride, a nitrate, a sulfate, and hydrates thereof formed using the metals. More particularly, the metal salt can be calcium chloride dihydrate (CaCl₂.2H₂O) and magnesium chloride hexahydrate (MgCl₂.6H₂O).

That is, in an embodiment of the present invention, the metal ion solution can be a binary metal ion solution including calcium ions (Ca²⁺) and magnesium ions (Mg²⁺). Here, a molar ratio of the calcium ions (Ca²⁺) to the magnesium ions (Mg²⁺) can be 1:1 to 1:20, preferably 1:1 to 1:15, more preferably 1:1 to 1:12.

A precipitation reaction does not occur even when the metal ion solution is added to the acidic water glass solution prepared in step 1 and, accordingly, silica of the water glass solution can be uniformly mixed with the metal in a solution phase, thereby improving reaction uniformity. This can be a result obtained by first adding the acid catalyst in step 1 to adjust the pH of the first water glass solution.

In addition, the metal ion solution can be added in an amount at which the metal ions in the solution can easily react with the water glass in the water glass solution. In particular, a volume ratio of the metal ion solution to the water glass solution can be 5:1 to 1:5. More particularly, a volume ratio of the metal ion solution to the water glass solution can be 3:1 to 1:3, even more particularly 2:1 to 1:2.

In addition, the metal ion solution can be added in an amount at which a molar ratio of silicon (Si) in the water glass solution to metal ions (Mg²⁺ and Ca²⁺) in the metal ion solution is 1:0.3 to 1:2.5.

The process of adding and mixing the metal ion solution is not specifically limited and, for example, can be a process of mixing through stirring. The stirring can be performed using a magnetic bar or mechanical mixer.

In the method according to an embodiment of the present invention, step 3 is a step of gelating the precursor solution. A silica precursor can be further added to the precursor solution of step 2, followed by a gelation reaction. In addition, the process of bubbling with an inert gas, performed in step 2, can be continuously performed in step 3.

Here, the inert gas includes the same gases as those described in step 2, preferably a nitrogen gas.

By performing bubbling with an inert gas in step 3, excessive growth of primary particles of the silica and metal oxide in the first and second water glass solutions during the gelation reaction can be limited. Accordingly, the intra-particle network structure can be further strengthened. Particles, the network structure of which is strengthened, show a very uniform pore distribution and high particle strength, which can improve the rate of particle size reduction caused by breakage and distortion of particles due to shear pressure. That is, when the metal oxide-silica composite aerogel of the present invention is applied as an additive for a polymer resin, collapse of the structure thereof due to shear pressure can be suppressed although the metal oxide-silica composite aerogel is subjected to a molding process such as extrusion or injection-molding along with the polymer resin. Accordingly, the metal oxide-silica composite aerogel can be usefully used as an additive.

Here, the primary particles can indicate metal oxide-silica composite aerogel particles starting to form as the gelation reaction proceeds.

In an embodiment of the present invention, a silicon-containing alkoxide-based compound such as tetramethyl orthosilicate (TMOS), tetraethyl orthosilicate (TEOS), or methyl triethyl orthosilicate can be used as the silica precursor. More particularly, water glass can be used. That is, a second water glass solution is preferably added to the precursor solution to perform a gelation reaction.

Here, the second water glass solution can function as a basic catalyst as well as a silica precursor.

As such, in the present invention, a high concentration second water glass solution is included, without separately using a basic catalyst used to form an aerogel, thereby preparing a metal oxide-silica composite aerogel. Accordingly, production costs can be reduced, and a metal oxide-silica composite aerogel can be efficiently prepared due to a simplified process.

Meanwhile, in the gelation reaction of step 3, an impregnation reaction of metal ions can occur while forming a gelated material with a water glass solution. For example, the formation of a gelated material and an impregnation reaction can be induced by adding a high concentration second water glass solution to increase pH and thus form a basic condition.

The concentration of the second water glass solution can be 0.5 M to 6.0 M, preferably 1.0 M to 5.0 M, more preferably 1.2 M to 4.0 M. Here, the concentration of the second water glass solution means the concentration of water glass in the second water glass solution. That is, the second water glass solution can include water glass in the concentration range, and the concentration of water glass in the second water glass solution can be higher than the concentration of water glass in the first water glass solution.

In addition, when the concentration of the second water glass solution is less than 0.5 M, the gelated material forms a porous network structure having excessively large pores, and contraction thereof increases during drying, whereby a metal oxide-silica composite aerogel having poor properties such as a very small pore volume, very poor pore uniformity, and a high effective density of single particles can be formed. When the concentration of the second water glass solution is higher than 6.0 M, the gelated material may not form a porous network structure and may form a dense structure, whereby the specific surface area of a finally prepared metal oxide-silica composite aerogel may be deteriorated. In addition, basicity may rapidly increase, whereby a gelation reaction may rapidly occur and, accordingly, a non-uniform pore structure may be formed. Further, a porous network structure having excessively large pores may be formed, whereby a metal oxide-silica a composite aerogel having significantly low pore characteristics may be formed, as described above.

In addition, the second water glass solution can be added so that the pH of the precursor solution is 3 to 10, preferably 3.5 to 9.5, more preferably 4 to 8.

When the pH of the precursor solution is outside the range, gelation may not be easily performed, or processability may be decreased due to an excessively low gelation rate.

Here, a slow gelation reaction can be induced by slowly adding the second water glass solution, and the second water glass solution can be preferably added in a continuous manner or in a manner of repeatedly, periodically adding several times, but the present invention is not limited thereto. However, a method of adding the second water glass solution batchwise can induce a non-uniform reaction due to insufficient mixing between reaction solutions.

In addition, since the pH of the precursor solution prepared in step 2 is within a low pH range and previously includes a water glass solution as a component thereof, the precursor solution of the present invention is not immediately gelated although the second water glass solution is added and gelation as a whole is slowly performed while the second water glass solution is uniformly mixed, unlike a conventional manner wherein gelation is immediately performed around a position at which a water glass solution is added when the water glass solution is directly added to a metal ion solution.

As such, since a slow gelation reaction is induced by slowly adding the second water glass solution in the present invention, a network structure of a gelated material can be more uniformly formed.

In addition, the gelation reaction is not specifically limited and, for example, can be performed while stirring. The stirring can be performed at 10 rpm to 500 rpm, preferably 100 rpm to 300 rpm, with a magnetic bar or a mechanical mixer. The gelation reaction can be performed for 30 minutes to 5 hours, preferably 1 to 4 hours, more preferably 1 to 3 hours.

In step 3, a basic catalyst can be further included although a high concentration second water glass solution is added. The basic catalyst can be used in an amount at which the pH of the precursor solution is 3 to 10, preferably 3.5 to 9.5, more preferably 4 to 8.

The basic catalyst is not specifically limited and can be, for example, one or more of sodium hydroxide (NaOH), ammonia water (NH₄OH), and potassium hydroxide (KOH). Particularly, the basic catalyst can be sodium hydroxide.

The method according to an embodiment of the present invention can further include a step of washing the prepared metal oxide-silica composite wet gel. The washing can be performed to obtain a high-purity metal oxide-silica composite aerogel by removing impurities (e.g., an unreacted substance such as Si⁴⁺, Mg²⁺, or Ca²⁺, a byproduct such as Na⁺ or Cl⁻, etc.) generated during reaction. The washing is not specifically limited and can be performed by a general method known in the art.

For example, the washing can be performed by adding distilled water or an organic solvent to the metal oxide-silica composite wet gel and stirring for 20 minutes to 1 hour. The organic solvent can be, without being specifically limited thereto, for example, an alcohol-based compound, a hydrocarbon-based compound, a ketone-based compound, or the like. More particularly, the organic solvent can be an alcohol-based compound such as methanol, ethanol, isopropanol, or propanol; a hydrocarbon-based compound such as hexane, octane, n-decane, n-heptane, n-undecane, cyclohexane, or toluene; a ketone-based compound such as methyl ethyl ketone or acetone; or a mixture of two or more thereof. When the washing is performed using an organic solvent, the organic solvent can be an alcohol-based compound which has excellent miscibility with water as a reactive solvent, can easily penetrate into pores inside silica gel particles, and exhibit a drying effect when the washing is combined with a subsequent drying process and resolve a pore contraction and deformation problem due to the drying process, and can be more particularly ethanol. When the alcohol-based compound is used, moisture present in the metal oxide-silica composite wet gel is replaced with an alcohol having relatively low surface tension, whereby contraction occurring during drying can be further suppressed.

The washing process can be performed according to a general washing method and can be repeated once or twice or more, particularly three to five times. When the washing process is performed twice or more, the same washing solvent or different washing solvents can be used. Particularly, a second washing process can be performed with a washing solvent including one or a mixture of two or more selected from the group consisting of methanol, tetrahydrofuran, toluene, and hexane after first washing with distilled water or ethanol.

In addition, the washing process of the method according to an embodiment of the present invention can further include a process of controlling a water content through solid/liquid separation.

The water content control process can be performed according to a general solid/liquid separation method using a vacuum filter or the like. More particularly, the water content control process can be performed so that a water content in a metal oxide-silica composite precipitate is 99% by weight or less, preferably 95% by weight or less, based on a total weight of the metal oxide-silica composite precipitate. By performing the water content control process through solid/liquid separation in this manner, a salt in and/or outside pores of the metal oxide-silica composite aerogel, as a final product, can be effectively removed, and processability can be increased while shortening a drying time.

The method according to an embodiment of the present invention can further include a step of drying the washed metal oxide-silica composite wet gel.

The drying step can be performed by a method such as heat treatment or hot air injection. A particular temperature and time in the drying step can be suitably adjusted depending upon a washing solvent type. Particularly, the drying step can be performed at 90° C. to 200° C. and can be performed under atmospheric pressure. The drying step can be performed while adjusting temperature, time, and the like such that a water content in the metal oxide-silica composite aerogel is 10% or less.

Meanwhile, “atmospheric pressure” refers to normal pressure, particularly a pressure which is not lowered or increased, and a separate high-pressure apparatus, such as an autoclave, is not used.

The method according to an embodiment of the present invention can include a step of preparing an acidic water glass solution, a step of adding a metal ion solution to the acidic water glass solution and uniformly mixing the same to prepare a precursor solution, and a step of slowly adding a high concentration second water glass solution to the precursor solution to induce a slow gelation reaction. Accordingly, a network structure and/or a uniform pore structure can be easily formed, whereby pore characteristics such as pore uniformity, a pore volume, a pore size, an effective pore volume, and a single particle effective density, a specific surface area, and a density reduction rate can be significantly improved. Accordingly, a metal oxide-silica composite aerogel capable of realizing an ultra-light weight can be prepared.

In addition, the present invention provides a metal oxide-silica composite aerogel prepared by the method.

A metal oxide of the aerogel according to an embodiment of the present invention can be doped with silica. The metal oxide is not specifically limited so long as it can be fixed by a silanol group on a surface of a silica aerogel to form a composite aerogel. The metal oxide can include particularly one or more metals selected from the group consisting of alkali metals, alkaline earth metals, lanthanides, actinides, transition metals, and metals in Group 13 (IIIA), more particularly one or more metal elements selected from the group consisting of calcium (Ca), magnesium (Mg), copper (Cu), zinc (Zn), manganese (Mn), cadmium (Cd), lead (Pb), nickel (Ni), chromium (Cr), silver (Ag), titanium (Ti), vanadium (V), cobalt (Co), molybdenum (Mo), tin (Sn), antimony (Sb), strontium (Sr), barium (Ba), and tungsten (W). Even more particularly, the metal oxide can be magnesium oxide, calcium oxide, or a mixture thereof. That is, the metal oxide-silica composite aerogel according to an embodiment of the present invention can include magnesium oxide (MgO), calcium oxide (CaO), and silica (SiO₂).

Here, the doping is a process of adding a limited amount of foreign material to a pure material. For example, the doping can be a process of bonding a metal oxide into a network structure of silica.

In addition, the pore uniformity of the metal oxide-silica composite aerogel can be represented by a full width at half maximum (FWHM) value. The FWHM value can be 7 nm or less, preferably 0.05 to 5 nm, more preferably 0.05 to 2 nm.

Here, the FWHM value, as a measure of pore uniformity, means an absolute value of a difference between two independent variable values which are half of a peak value in a pore diameter-dependent distribution, as shown in FIGS. 3 and 4.

A smaller FWHM value indicates that pore diameter distribution of the metal oxide-silica composite aerogel is narrow and thus a pore size is more uniform. On the other hand, a larger FWHM value indicates that pore particle diameter distribution is broad and thus a pore size is relatively non-uniform. Accordingly, an FWHM value in a pore diameter-dependent distribution represents the uniformity of a pore size, i.e., pore uniformity.

While a conventional aerogel has an FWHM value of 40 nm or more, a metal oxide-silica composite aerogel prepared by the method according to an embodiment of the present invention can have an FWHM value of 7 nm or less, most preferably 2 nm or less. Accordingly, the present invention can provide a metal oxide-silica composite aerogel having significantly improved pore uniformity.

In addition, the metal oxide-silica composite aerogel prepared by the method according to an embodiment of the present invention can have a pore volume of 0.9 cm³/g or more, preferably 1 to 3 cm³/g, more preferably 1.3 to 2.5 cm³/g.

In particular, the metal oxide-silica composite aerogel can have an effective pore volume of 0.9 cm³/g or more, preferably 1 to 3 cm³/g, more preferably 1.3 to 2.5 cm³/g, thereby having an effective pore volume that is not greatly different from the aforementioned pore volume. That is, a ratio of an effective pore volume can be 0.9 or more, preferably 0.98 to 1, based on a measured total pore volume, which may indicate that the entirety or most of a pore volume in the metal oxide-silica composite aerogel is an effective pore volume.

Here, the effective pore volume may mean the volume of pores, into which penetration of a polymer resin is limited, when the metal oxide-silica composite aerogel is used as an additive for weight reduction. Particularly, the effective pore volume can be obtained from a cumulative pore volume at a pore size of about 50 nm or less. Since penetration of a polymer resin into pores occupying the effective pore volume is limited, a weight reduction property is superior as an effective pore volume and a ratio of the effective pore volume to a total pore volume are large.

While a conventional aerogel has a low effective pore volume of 0.5 to 0.8 cm³/g, the metal oxide-silica composite aerogel prepared by the method according to an embodiment of the present invention can have an effective pore volume of 0.9 cm³/g or more, most preferably 1.3 to 2.5 cm³/g. Accordingly, a metal oxide-silica composite aerogel having an excellent weight reduction property can be provided.

The metal oxide-silica composite aerogel prepared by the method according to an embodiment of the present invention can have a single particle effective density of 0.7 g/ml or less, preferably 0.01 to 0.7 g/ml, more preferably 0.01 to 0.6 g/ml.

Here, the single particle effective density refers to the density of an aerogel substantially affecting weight reduction, except for a part into which a polymer resin penetrates, when an aerogel is applied as an additive and can be obtained according to Equation 2 below:

$\begin{matrix} {\left( {{Single}\mspace{14mu}{Particle}\mspace{14mu}{Effective}\mspace{14mu}{Density}} \right) = \left\lbrack {\frac{1}{\left( {{Skeletal}\mspace{14mu}{Density}} \right)} + \left( {{Effective}\mspace{14mu}{Pore}\mspace{14mu}{Volume}} \right)} \right\rbrack^{- 1}} & \left\lbrack {{Equation}\mspace{14mu} 2} \right\rbrack \end{matrix}$

-   -   Single particle effective density: Effective density of single         particles     -   Skeletal density: Density of metal oxide-silica composite         aerogel itself     -   Effective pore volume: Cumulative pore volume of pores having a         size of 50 nm or less in a metal oxide-silica composite aerogel

Here, a lower single particle effective density means that the number of pores into which a polymer resin does not penetrate is large and thus a weight reduction property is superior. While a conventional aerogel has a single particle effective density of about 0.9 g/ml and thus poor applicability as an additive for reducing the weight of a polymer resin, the metal oxide-silica composite aerogel prepared by the method according to an embodiment of the present invention has a single particle effective density of 0.7 g/ml or less, most preferably 0.01 to 0.6 g/ml. Accordingly, the present invention can provide an aerogel that has an excellent weight reduction property and is capable of being used as an additive for reducing the weight of a polymer resin.

Here, the density of the metal oxide-silica composite aerogel itself can be measured by means of a gas pycnometer (AccuPyc 1340, manufactured by Micromeritics) at room temperature under atmospheric pressure.

In the present invention, “room temperature” can be a temperature that is not separately lowered or increased. In general, room temperature can be 20±5° C.

In addition, the metal oxide-silica composite aerogel prepared by the method according to an embodiment of the present invention can have a density reduction rate of 3% or more, preferably 4 to 20%, more preferably 5 to 20%.

Here, the density reduction rate can refer to a ratio (percentage) of a density, which is reduced by adding a predetermined amount of metal oxide-silica composite aerogel to a polypropylene resin, to the density of the polypropylene (PP) resin itself. That is, as a density reduction rate is large, the number of effective pores, into which a polymer resin does not penetrate, is large, which indicates a superior weight reduction property.

In addition, a pore diameter of the metal oxide-silica composite aerogel prepared by the method according to an embodiment of the present invention is not specifically limited and can be preferably 50 nm or less. When the pore diameter of the metal oxide-silica composite aerogel is larger than 50 nm, a polymer resin can penetrate into pores of the metal oxide-silica composite aerogel due to the large pores, whereby the weight reduction property of the metal oxide-silica composite aerogel can be deteriorated.

In addition, the metal oxide-silica composite aerogel can have a specific surface area of 500 m²/g or more, preferably 600 m²/g to 1500 m²/g, more preferably 800 m²/g to 1500 m²/g.

In the present invention, the pore volume, the pore diameter, and the specific surface area can be a value analyzed by an adsorption/desorption amount of nitrogen according to a partial pressure (0.11<p/p₀<1) by feeding 0.3 g of a sample, moisture and organic ingredients of which have been removed by performing pre-treatment at 150° C. under vacuum for 24 hours, into a BET measurer (ASAP 2010, manufactured by Micrometrics). Here, the specific surface area can refer to a BET specific surface area.

In addition, the metal oxide-silica composite aerogel according to an embodiment of the present invention can have a particle diameter change rate of 10% or less, preferably 7% or less, more preferably 5.5% or less. The particle diameter change rate can be calculated according to Equation 1 below. A smaller particle diameter change rate indicates stronger particle strength. In addition, when particle strength is large, cracking of particles can be prevented when the metal oxide-silica composite aerogel is processed through injection-molding, extrusion, and the like. Accordingly, the metal oxide-silica composite aerogel can be easily processed as an additive for weight reduction.

$\begin{matrix} {{{Particle}\mspace{14mu}{diameter}\mspace{14mu}{change}\mspace{14mu}{rate}\mspace{11mu}(\%)} = {\left\lbrack {1 - \frac{D_{50}\left( {1\mspace{14mu}{bar}} \right)}{D_{50}\left( {0\mspace{14mu}{bar}} \right)}} \right\rbrack \times 100(\%)}} & \left\lbrack {{Equation}\mspace{14mu} 1} \right\rbrack \end{matrix}$

Here, D₅₀, which is an average particle diameter, can be defined as a particle diameter corresponding to 50% of a cumulative number in a particle size distribution curve of particles. In addition, the particle diameter of the present invention can be measured at room temperature by means of a particle size analyzer (S3500, manufactured by Microtrac).

In addition D₅₀ (1 bar) is an average particle diameter when a compression shear pressure is 1 bar, and D₅₀ (0 bar) is an average particle diameter when a compression shear pressure is 0 bar.

The metal oxide-silica composite aerogel according to an embodiment of the present invention prepared by the aforementioned method has an increased specific surface area, thus exhibiting porosity. In addition, the metal oxide-silica composite aerogel has excellent pore uniformity and improved pore characteristics such as an effective pore volume and a single particle effective density, whereby many effective pores into which a polymer resin cannot penetrate are present therein. Accordingly, the metal oxide-silica composite aerogel has an excellent weight reduction property.

Hereinafter, the present invention will be described in more detail with reference to the following examples and experimental examples. However, the examples and experimental examples are provided to exemplify the present invention, and the scope of the present invention is not limited thereto.

Example 1

A metal oxide-silica composite aerogel was prepared through the steps shown in FIG. 2.

In particular, hydrochloric acid (acid catalyst) was added to 200 ml of a 0.25 M water glass solution (first water glass solution) to prepare an acidic water glass solution. Here, the pH of the acidic water glass solution was 0.9. The acidic water glass solution was bubbled with a N₂ gas, and 200 ml of a 0.05 M metal ion solution having a molar ratio of Mg²⁺:Ca²⁺=9:1 was added to and stirred with the acidic water glass solution, thereby preparing a precursor solution. Here, the concentration of the precursor solution was 2 wt %. Subsequently, a gelation reaction was performed for 2 hours while slowly adding a 2.0 M water glass solution (second water glass solution) to the precursor solution until the pH of the precursor solution became 8.0, thereby preparing a metal oxide-silica composite wet gel. Here, the bubbling was continuously performed when the gelation reaction was performed. The prepared metal oxide-silica composite wet gel was subjected to solid/liquid separation to a water content of 95% or less while washing the same with a washing solvent. The washing was repeated four times. The washed metal oxide-silica composite wet gel was dried in a 125° C. oven for 6 hours under atmospheric pressure, thereby preparing a metal oxide-silica composite aerogel.

Example 2

A metal oxide-silica composite aerogel was prepared in the same manner as in Example 1, except that the concentration of a precursor solution was adjusted to 1 wt % by mixing a 0.125 M first water glass solution with a 0.025 M metal ion solution, and a 1.0 M second water glass solution was added.

Example 3

A metal oxide-silica composite aerogel was prepared in the same manner as in Example 1, except that the concentration of a precursor solution was adjusted to 2.5 wt % by mixing a 0.313 M first water glass solution with a 0.063 M metal ion solution, and a 2.5 M second water glass solution was added.

Example 4

A metal oxide-silica composite aerogel was prepared in the same manner as in Example 1, except that the concentration of a precursor solution was adjusted to 3 wt % by mixing a 0.375 M first water glass solution with a 0.075 M metal ion solution, and a 3 M second water glass solution was added.

Example 5

A metal oxide-silica composite aerogel was prepared in the same manner as in Example 1, except that hydrochloric acid was added so that the pH of a first water glass solution became 0.85, and a second water glass solution was added until the pH of the acidic reactant solution became 6.0.

Comparative Example 1

A metal oxide-silica composite aerogel was prepared in the same manner as in Example 1, except that a step of bubbling with a N₂ gas was omitted.

Comparative Example 2

A metal oxide-silica composite aerogel was prepared in the same manner as in Example 1, except that a step of bubbling with a N₂ gas was omitted, the pH of the acidic water glass solution was adjusted to 1.1, and a second water glass solution was added until a pH became 9.5.

Comparative Example 3

Hydrochloric acid (acid catalyst) was added to 200 ml of a 0.5 M water glass solution until a pH became 7, and a gelation reaction was performed, thereby preparing a silica wet gel. 200 ml of a 0.25 M metal ion solution at pH 0.9, which included an acid catalyst and had a molar ratio of Mg²⁺:Ca²⁺=9:1, and a 2.0 M water glass solution were added to the silica wet gel until a pH became 7.5, and a second gelation reaction was performed, thereby preparing a metal oxide-silica composite wet gel. Washing and drying were performed in the same manner as in Example 1. As a result, a metal oxide-silica composite aerogel was prepared.

Comparative Example 4

Hydrochloric acid was added to 200 ml of a 0.5 M water glass solution until a pH became 4, and a gelation reaction was performed, thereby preparing a silica wet gel. 200 ml of a 0.25 M metal ion solution having a molar ratio of Mg²⁺:Ca²⁺=9:1 and a 2.0 M water glass solution were added to the silica wet gel until a reaction pH became 7.5, and a second gelation reaction was performed, thereby preparing a metal oxide-silica composite wet gel. Washing and drying were performed in the same manner as in Example 1. As a result, a metal oxide-silica composite aerogel was prepared.

Comparative Example 5

Hydrochloric acid was added to 200 ml of a 0.25 M metal ion solution having a molar ratio of Mg²⁺:Ca²⁺=9:1 until a pH became 4.0, thereby preparing an acidic metal ion solution. A 2.0 M water glass solution was slowly added to the acidic metal ion solution until a reaction pH became 7.5, and a precipitation reaction was performed for 2 hours, thereby preparing a metal oxide-silica composite wet gel. Washing and drying were performed in the same manner as in Example 1. As a result, a metal oxide-silica composite aerogel was prepared.

Experimental Example

To compare the properties of the metal oxide-silica composite aerogels prepared in Examples 1 to 5 and Comparative Examples 1 to 5, specific surface area (BET, m²/g), pore volume (cm³/g), pore diameter (nm), effective pore volume (cm³/g), and compression shear-dependent particle size changes of each of the metal oxide-silica composite aerogels were measured. Pore uniformity (FWHM, nm) and single particle effective density (g/ml) were calculated using the measured values. Results are summarized in Table 1 below.

1) Specific Surface Area (BET, m²/g), Pore Volume (V_(pore), Cm³/g), and Average Pore Diameter (D_(pore), Nm)

A specific surface area, a pore volume, and an average pore diameter were measured by an adsorption/desorption amount of nitrogen according to a partial pressure (0.11<p/p₀<1) using an ASAP 2010 apparatus (manufactured by Micromeritics). In particular, 0.3 g of a sample, moisture and organic ingredients of which have been removed by performing pre-treatment at 150° C. under vacuum for 24 hours, was fed into a BET measurer (ASAP 2010, manufactured by Micrometrics) to measure a specific surface area, a pore volume, and an average pore diameter.

2) Pore Uniformity (FWHM, Nm)

As shown in FIGS. 3 to 5, FWHM was calculated as an absolute value of a difference between two independent variable values which are half of a peak value in a pore diameter-dependent distribution.

3) Effective Pore Volume (Cm³/g)

Effective pore volume refers to a total volume of pores having a pore diameter of 50 nm or less at the pore volume measured in 1).

4) Single Particle Effective Density (g/ml)

After measuring the density of a metal oxide-silica composite aerogel itself (skeletal density) at room temperature under atmospheric pressure by means of a gas pycnometer (AccuPyc 1340, manufactured by Micromeritics), a single particle effective density was calculated according to Equation 2 below using an effective pore volume measured in 3).

$\begin{matrix} {\left( {{Single}\mspace{14mu}{Particle}\mspace{14mu}{Effective}\mspace{14mu}{Density}} \right) = \left\lbrack {\frac{1}{\left( {{Skeletal}\mspace{14mu}{Density}} \right)} + \left( {{Effective}\mspace{14mu}{Pore}\mspace{14mu}{Volume}} \right)} \right\rbrack^{- 1}} & \left\lbrack {{Equation}\mspace{14mu} 2} \right\rbrack \end{matrix}$

-   -   Single particle effective density: Effective density of single         particles     -   Skeletal density: Density of metal oxide-silica composite         aerogel itself     -   Effective pore volume: Cumulative pore volume of pores having a         size of 50 nm or less in a metal oxide-silica composite aerogel

5) Particle Diameter Change Rate

Particle diameters were measured at room temperature using a particle size analyzer (S3500, manufactured by Microtrac). Particle diameters under a compression shear pressure of 0 bar and particle diameters under a compression shear pressure of 1 bar were measured, and a particle diameter change rate was calculated according to Equation 1 below:

$\begin{matrix} {{{Particle}\mspace{14mu}{diameter}\mspace{14mu}{change}\mspace{14mu}{rate}\mspace{11mu}(\%)} = {\left\lbrack {1 - \frac{D_{50}\left( {1\mspace{14mu}{bar}} \right)}{D_{50}\left( {0\mspace{14mu}{bar}} \right)}} \right\rbrack \times 100(\%)}} & \left\lbrack {{Equation}\mspace{14mu} 1} \right\rbrack \end{matrix}$

Here, D₅₀, which is an average particle diameter, can be measured, for example, using a particle size analyzer, and an average particle diameter (D₅₀) of particles can be defined as a particle diameter on the basis of 50% of a cumulative number in a particle diameter distribution.

In addition D₅₀ (1 bar) is an average particle diameter when a compression shear pressure is 1 bar, and D₅₀ (0 bar) is an average particle diameter when a compression shear pressure is 0 bar.

Table 1 below shows main reaction conditions and experimental results of Examples 1 to 5 and Comparative Examples 1 to 5.

TABLE 1 Compar- Compar- Compar- Compar- Compar- ative ative ative ative ative Exam- Exam- Exam- Exam- Exam- Exam- Exam- Exam- Exam- Exam- ple 1 ple 2 ple 3 ple 4 ple 5 ple 1 ple 2 ple 3 ple 4 ple 5 Main N₂ N₂ N₂ N₂ N2 N₂ N₂ Highly Highly Highly reaction Bubbling Bubbling Bubbling Bubbling Bubbling Bubbling Bubbling porous porous porous condition omitted omitted MSC MSC MSC pH 8.0 8.0 8.0 8.0 6.0 8.0 9.5 7.5 7.5 7.5 BET 840 980 750 740 940 550 310 380 270 500 (m²/g) Pore 1.23 1.52 1.12 1.08 1.05 1.04 1.14 0.98 1.12 0.40 volume (cm³/g) Pore 7.17 5.64 8.17 8.84 4.99 9.80 22.01 15.20 16.56 8.42 diameter (nm) Pore 0.94 0.54 1.17 1.38 1.03 2.48 4.63 5.62 54.50 — uniformity (Calcu- (FWHM, lation nm) impos- sible) Effective 1.23 1.52 1.12 1.08 1.05 1.04 1.14 0.98 0.82 0.07 pore volume (cm³/g) Single 0.61 0.51 0.64 0.65 0.64 0.65 0.63 0.68 0.75 1.32 particle effective density (g/ml) Particle 3.3 4.0 4.8 5.1 3.8 11.0 12.4 62.3 70.1 53.8 diameter change rate (%) * BET: BET specific surface area * P.V.: Pore volume * MSC: Metal oxide-silica composite aerogel

As shown in Table 1, it can be confirmed that the metal oxide-silica composite aerogels of Examples 1 to 5 prepared according to the method according to an embodiment of the present invention exhibit a large specific surface area, a large pore volume, and, particularly, remarkably improved pore uniformity (FWHM) and a remarkably improved effective pore volume, compared to the metal oxide-silica composite aerogels of Comparative Examples 1 to 5. Accordingly, it can be confirmed that the metal oxide-silica composite aerogels of Examples 1 to 5 exhibit an excellent single particle effective density, density reduction rate and effective pore volume relative to a pore volume. Further, it can be confirmed that the metal oxide-silica composite aerogels of Examples 1 to 5 have smaller particle diameter change rates due to compression shear pressure, thus having higher particle strength.

In particular, comparing Examples 1 to 5 to Comparative Examples 3 to 5, it can be confirmed that Examples 1 to 5 exhibit large specific surface areas on average, compared to Comparative Examples 3 to 5. This indicates that the metal oxide-silica composite aerogels of Examples 1 to 5 are more porous, compared to those of Comparative Examples 3 to 5. This result can be re-confirmed from that result that the metal oxide-silica composite aerogels of Examples 1 to 5 have higher pore volumes, compared to those of Comparative Examples 3 to 5. Further, it can be confirmed that the metal oxide-silica composite aerogels of Examples 1 to 5 exhibit a cumulative pore volume at a pore size of 50 nm or less into which it is difficult for a polymer to penetrate, which indicates that the effective pore volumes, substantially affecting a weight reduction property, of Examples 1 to 5 are significantly superior to those of Comparative Examples 3 to 5. Accordingly, the metal oxide-silica composite aerogels of Examples 1 to 5 have remarkably reduced single particle effective densities, compared to Comparative Examples 3 to 5. These results indicate that an aerogel prepared according to the method of the present invention is very useful as an additive for weight reduction.

In addition, Examples 1 to 5 exhibit remarkably increased pore uniformity, compared to Comparative Examples 3 to 5. Form the increased pore uniformity, it can be deduced that the metal oxide-silica composite aerogels of Examples 1 to 5 have uniform pore structures, whereby contraction thereof during drying is reduced.

These results can be re-confirmed from FIGS. 4 and 5 of the present invention. FIG. 4 shows a pore diameter-dependent distribution of Example 1 and Comparative Examples 1 to 5, and FIG. 5 is a view illustrating a pore diameter-dependent distribution enlarged based on the comparative examples of FIG. 4. Examining FIGS. 4 and 5, it can be confirmed that the metal oxide-silica composite aerogel of Example 1 has a very narrow pore diameter distribution curve and a pore diameter peak formed at a very small numerical value, whereas the metal oxide-silica composite aerogels of Comparative Examples 3 to 5 prepared respectively using conventional methods all have broad pore diameter distribution curves and, in the case when peaks are formed, have two or more peaks. In particular, it can be confirmed that, in Comparative Example 5, a very broad distribution curve is formed so that an FWHM value of the curve is not calculated.

In addition, as shown in Table 1, the FWHM value indicating pore uniformity is calculated as a very large numerical value in Comparative Example 4. However, this FWHM value is inaccurate because the peak is not normally formed.

Accordingly, it can be confirmed that the pore uniformity of the metal oxide-silica composite aerogel of Example 1 of the present invention is significantly superior to those of Comparative Examples 3 to 5, and effective pores, into which penetration of a polymer resin is limited due to small pore diameters, occupy most of the entire pores of the metal oxide-silica composite aerogel.

In addition, since, in Comparative Examples 3 to 5, a step of mixing metal ions with a water glass solution is omitted and gelation immediately occurs as soon as metal ions contact the water glass solution or the metal ion solution is mixed in a wet gel state to prepare a metal oxide-silica composite aerogel, unlike Examples 1 to 5, the metal is relatively non-uniformly present in the aerogel, compared to Examples 1 to 5. Accordingly, when the metal oxide-silica composite aerogels prepared according to Comparative Examples 3 to 5 are used as additives, it can be confirmed from the particle diameter change rate results that mechanical strength is decreased and it can confirmed that pore characteristics as a whole are decreased due to a non-uniform network structure of the metal and the silica. In particular, Comparative Examples 3 to 5 exhibit greatly deteriorated mechanical strength, as shown in Table 1, even though a significantly high concentration metal ion solution is used compared to Examples 1 to 5. On the other hand, since the precursor solution, in which the water glass solution and the metal ion solution are uniformly mixed, is formed before gelation, unlike conventional metal oxide-silica composite aerogels, in the present invention, the metal can be uniformly present in the aerogel also after gelation. Accordingly, it can be confirmed that, although a low concentration metal ion solution is used, a gelated structure can be satisfactorily strengthened and mechanical strength can also be improved.

In addition, when the metal oxide-silica composite aerogels of Examples 1 to 5 are compared with the metal oxide-silica composite aerogels of Comparative Examples 1 and 2 prepared in a similar manner to those of Examples 1 to 5 except that a step of bubbling with a N₂ gas was omitted, it can be confirmed that Examples 1 to 5 exhibit a decrease in a particle diameter change rate due to compression shear pressure, compared to Comparative Examples 1 and 2. This result indicates that the particle strength of the metal oxide-silica composite aerogels of Examples 1 to 5 is greater than that of Comparative Examples 1 and 2. That is, since the metal oxide-silica composite aerogels of Examples 1 to 5 have greater particle strength, structural collapse due to shear pressure occurring during extrusion or injection-molding of a resin can be prevented when applied as an additive to a polymer resin. Accordingly, the metal oxide-silica composite aerogels of Examples 1 to 5 can be usefully used as an additive for weight reduction of a polymer resin.

These results can be confirmed in FIG. 4 showing a pore diameter-dependent distribution of Example 1 and Comparative Examples 1 to 5. From FIG. 4, it can be confirmed that Example 1 including a step of bubbling with a N₂ gas exhibits excellent pore uniformity due to a narrow pore distribution and pore diameter peaks formed at small numerical values, compared to Comparative Examples 1 to 5, as described above. From these results, it can be assumed that a relatively large number of effective pores are present in the metal oxide-silica composite aerogels of Examples 1 to 5.

In addition, when Example 4 is compared with Comparative Example 1, it can be confirmed that, although the concentration of the precursor of Example 4 is high, the pore uniformity of Example 4 is improved by performing the bubbling step.

From the experimental results, it can be confirmed that, in preparing the metal oxide-silica composite aerogel of the present invention, the metal ion solution is mixed in an acidic water glass solution state such that precipitation does not occur, and a high concentration water glass solution is slowly gelated to strengthen the internal network structure so that the pore volume increases, an effective pore volume and an effective density are improved, and a uniform pore structure is formed. Accordingly, an aerogel useful as an additive for weight reduction can be prepared. Further, it can be confirmed that, due to inclusion of the step of bubbling with a nitrogen gas upon addition of the metal ion solution, the growth of primary particles is inhibited and the network structure is further strengthened. Accordingly, it can be confirmed that an aerogel, in which a particle diameter change rate due to compression shear pressure is very low, can be prepared. 

The invention claimed is:
 1. A method of preparing a metal oxide-silica composite aerogel, the method comprising: 1) adding an acid catalyst to a first water glass solution to prepare an acidic water glass solution; 2) adding a metal ion solution to the acidic water glass solution while bubbling an inert gas into the acidic water glass solution to prepare a precursor solution having a reduced amount of dissolved oxygen; and 3) adding a second water glass solution to the precursor solution and performing a gelation reaction to yield a metal oxide-silica composite wet gel, wherein, in step 3), bubbling of an inert gas is performed during the adding of the second water glass solution.
 2. The method according to claim 1, wherein the inert gas comprises one or more of helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), radon (Rn), nitrogen (N₂), and carbon dioxide (CO₂) gases.
 3. The method according to claim 1, wherein the pH of the acidic water glass solution is 0.1 to 2.9.
 4. The method according to claim 1, wherein the concentration of the first water glass solution is 0.01 M to 2.0 M.
 5. The method according to claim 1, wherein the concentration of the metal ion solution is 0.01 M to 2.0 M.
 6. The method according to claim 1, wherein the concentration of the second water glass solution is 0.5 M to 6.0 M.
 7. The method according to claim 1, wherein the concentration of the second water glass solution is higher than the concentration of the first water glass solution.
 8. The method according to claim 1, wherein the second water glass solution is added such that the pH of the precursor solution becomes 3 to
 10. 9. The method according to claim 1, wherein the metal ion solution comprises one or more metal ions selected from the group consisting of alkali metals, alkaline earth metals, lanthanides, actinides, transition metals, and metals in Group 13 (IIIA).
 10. The method according to claim 1, wherein the metal ion solution is a binary metal ion solution comprising calcium ions (Ca²⁺) and magnesium ions (Mg²⁺).
 11. The method according to claim 10, wherein a molar ratio of the calcium ions (Ca²⁺) to the magnesium ions (Mg²⁺) in the metal ion solution is 1:1 to 1:20.
 12. The method according to claim 1, wherein the acid catalyst is one or more than one acid selected from the group consisting of hydrochloric acid, nitric acid, acetic acid, sulfuric acid, and hydrofluoric acid.
 13. The method according to claim 1, further comprising, after step 3, washing and drying the metal oxide-silica composite wet gel. 